Energetic neutral atom imaging at low altitudes from the Swedish microsatellite Astrid: Extraction of the equatorial ion distribution

Energetic neutral atom (ENA) images obtained by the imager on board the Astrid satellite in the polar cap at 1000 km during a moderate magnetic storm (Dst ≥ −80 nT) on February 8, 1995, are simulated using a parameterized model of the equatorial ion distribution and a six‐component Chamberlain exosphere with parameters from the Mass Spectrometer Incoherent Scatter Extended (MSISE‐90) model. By changing the ion parameters until a matching ENA image is obtained, one can extract the equatorial ion distribution. Four consecutive images from different view points several minutes apart are simulated assuming H+ and O+, respectively, as parent ions. The optimal set of parameters is extracted by minimizing the χ2 difference between simulated and observed ENA images using Powell's minimization algorithm. The optimal equatorial model ion distribution consists of O+ peaked around dusk. The lower intensity of fluxes obtained from vantage points closer to the pole is an effect of the loss cone of the parent ion distribution being empty.